This invention relates to electro-chemical gas sensors in which the gas or vapor to be sensed is caused to react at an electrode of an electrochemical device, thereby setting up a current through an electrolyte across the device, i.e. to a counter electrode such that the current is a function of the partial pressure of the gas to be sensed. In such cells the gas to be sensed, reaches the electrode through a diffusion path which limits the total amount of gas which actually reacts at the cell to a relatively small amount which amount, however, is a known function of, say, the concentration of gas in the atmosphere being monitored.
The prior art is replete various devices featuring diffusion-path structures ranging from thin solid membranes requiring solution of the gas in the membrane before it moves to the electrode to a Knudsen barrier type device disclosed in U.S. Pat. No. 4,324,632. U.S. Pat. No. 4,132,616 also shows a diffusion barrier which is, in large part, defined by a capillary in series with a porous body. The prior art devices are entirely suitable in performance, but it remained desirable to provide a sensor configuration that could be more readily adjusted in the field and more easily fine-tuned during manufacture.